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Body tissues

Body tissues grow by increasing the number of cells that make them up. The cells reproduce themselves exactly. One cell doubles by dividing into two. Two cells become four and so on.

Diagram showing how cells reproduce

This happens very fast between conception and adulthood. But once we are grown up, most cells only reproduce in order to replace others that have died, for example through injury or illness. Some cells carry on reproducing. These include sperm cells, hair cells, cells in the gut and cells that make blood in the bone marrow.

Not all cells carry on being able to reproduce. Most cells mature and become specialised for their particular job in the body. Mature cells may lose the ability to reproduce as they develop. But there will always be enough immature cells around (called stem cells) to replace cells that are damaged or killed.

 

How cells grow and reproduce

When more cells are needed by the body, some cells double up to increase their numbers. They do this very precisely so that the new cells are exactly the same as the old ones. Each cell makes copies of all its genes. Then it splits into two with one set of genes in each new cell. The video below shows a cell dividing in two.

If even more cells are needed, these new cells will rest for a while and then reproduce again. The cells will carry on doing this until enough cells have been made.

 

How cells know when to stop growing

Normal growth and healing is very orderly and precise. The cells know when there are enough new cells to mend a cut or when a structure such as a finger is fully grown. They send chemical messages to each other so that they stop growing and reproducing when growth or healing is complete. The diagram below shows this happening.

Diagram showing how cells know when to stop dividing

Diagram showing how cells know when to stop growing

Scientists are still finding out how the cells do this. The chemical messages come from the genes inside the cells. Some genes become switched on (activated) and tell cells to reproduce and other genes tell them not to.

 

How new cells end up in the right place

Why does a growing finger end up finger shaped? It seems the cells have a natural ability to stick together in the right place. Scientists call this cell adhesion.

Diagram showing how new cells end up in the right place

Molecules on the surface of the cell match those on its neighbours. It is a bit like having a post code. The code makes it very difficult for the cell to move to the wrong place. But if it does find itself in a place where its post code is different from its neighbours, it dies.

 

How long cells carry on doubling for

It seems that human cells are programmed to reproduce up to 50 or 60 times at most. Then they die.Stem cells provide a pool of dividing cells that the body uses to restock damaged or old cells.

Other things can make the cell self destruct before it gets as far as doubling 60 times. If the genes of the cell are very badly damaged, or it has broken away from its proper place, it will self destruct. This is called 'apoptosis'. Scientists are doing a lot of work on apoptosis. If they can understand what makes a cell self destruct, they might be able to use this to develop cancer treatments in the future.

 

The difference between cancer cells and normal cells

Cancer cells are different from normal cells in a number of very important ways. Look at our section on the cancer cell for more information about this.

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Updated: 24 July 2013